Go to your Zappiti Player 4k.
From Settings, scroll down to "About" section.
In "Build number", you'll find the currently installed firmware version number.
For instance : "rtk_phoenix-eng 4.4 KRT160 eng.tony.20151006"
The current version is then 20151006, October 6th 2015.
Now, go to this page : http://zappiti.com/zappiti_player_firmwares.html#z4k
In this page, scroll down to the latest firmware download link.
For instance : "Zappiti Player 4K Firmware version "20151103 OS2.3 b6 beta"... (.zip)"
Here, the version number is 20151103, November 3rd 2015.
If your currently installed firmware version is prior to the latest available, please follow the procedure described here to update the firmware.